Rub-resistance is that property of an ink-jet ink that allows the ink to be printed onto a substrate, and after aging, show no signs of smudging or an ink transfer when the printed substrate is rubbed. Water fastness is that property that allows water to be spilled on the printed page without smearing or otherwise destroying the image. These properties are not easy to obtain, and are particularly difficult for substrates such as waxy substrates, transparent plastic substrates, e.g., acetate substrates, and pre-screen print substrates wherein a color has already been printed as an overall background.
As with any property in ink-jet inks, rub resistance and water fastness are not readily obtained by the simple addition of one more chemical to the ink formulation. The reason is that there are a number of other properties that tend to be altered, when another chemical is added. One of the most important of these other properties is filterability. That is, one of the last steps in ink manufacture is filtration of the ink through one or more filters down to a pore size of 1.2 .mu.. The purpose is to eliminate extraneous matter usually introduced via the dye stuff. Failure to filter the finished ink to remove such matter usually leads to plugging of the printer orifices.
However, the very materials that can result in improved water fastness and rub resistance also tend to cause blockage of the filter. If the ink blocks the filter, it of course cannot be adequately filtered without continued replacement of the filters. Such replacements are extremely expensive.
It has been found that the following filterability test is a reliable measure of an ink's ability to pass through such filters:
The ink composition without the binder is first filtered down to 1.2 microns. (This is to remove the matter contributed by the dye stuff, so that the filterability of the binder can be ascertained without the dye stuff in fact being the cause of filter blockage. This first filtration step is omitted, of course, in actual manufacturing of the ink.)
Thereafter, the acrylic binder is added in an amount that insures that 1.37 weight % of the ink comprises the binder as nonvolatile solids. About one liter of this total ink is then filtered under vacuum of about 18 cm of Hg through, a) a glass fiber depth filter having nominal pore sizes of 1 micron; then b), a membrane filter having a pore size of 3.0 microns; and then c), a membrane filter having a pore size of 1.2 microns. All filters a)-c) are 47 mm in diameter. If no substantial blockage occurs in the filteration through any of the three filters a)-c), the ink has passed the test. As used herein, "no substantial blockage" means, the ink is able to flow through each of the filters without being reduced to a dripping flow. That is, all the ink comes through as a continuous stream.
U.S. Pat. No. 4,334,878 describes an ink-jet ink that purports to provide "smear resistance." Key components of this ink are ammonia and an acrylic binder which is a copolymer of acrylic acid and styrene. A specific preferred example is Joncryl 67 obtained from S.C. Johnson Co. However, this binder is base soluble, and has been found to in fact, provide insufficient rub-resistance.
Thus, what has been needed prior to this invention is an ink for ink-jet printers that provides rub resistance and water fastness, without sacrificing other essential properties such as filterability.